1. Field of the Invention
The present invention is in the field of degradation of organic contaminants, and more specifically is directed to a process for oxidatively degrading pentachlorophenol by a microbially driven Fenton reaction.
2. Description of the Related Art
Hazardous waste cleanup in soil and groundwater has gained increased attention recently due to the increasing number of waste disposal sites. Economic and environmental considerations spur research towards alternative remediation methods. Bioremediation has been studied as a possible alternative to other conventional waste treatment processes.
Pentachlorophenol (PCP) is a recalcitrant biocide widely used throughout the United States as a fungicide and wood preservative. Paper mill wastewater effluents and groundwaters are often contaminated with PCP, which is classified as a priority pollutant by the United States Environmental Protection Agency.
Research has focused on the conversion of PCP contained in soil and groundwater by on-site and in situ treatments, including incineration and bioremediation. Incineration has the disadvantage of formation of harmful by-products. Accordingly, a great deal of research has focused on improved remediation of organic components in soils, sediments, sludges and slurries.
Free radical-based oxidation technologies provide an attractive alternative to conventional treatment strategies for elimination of hazardous waste. These treatment technologies harness the oxidizing potential of hydroxyl radicals to degrade a wide variety of hazardous organic compounds. Hydroxyl radicals are among the most reactive oxidants found in aqueous environments and can readily oxidize a variety of naturally occurring and contaminating organic compounds. The hydroxyl initiates a cascade of oxidation reactions that can lead to total mineralization of organic pollutants.
Radical-based oxidation reactions have been used in the art in conjunction with biological treatment. U.S. Pat. No. 5,955,350 to Soni et al. discloses a process for remediation of contaminated solid materials containing polynuclear aromatic hydrocarbons, polychlorinated hydrocarbons and mixtures thereof by sequential biological/chemical/biological treatment. Soni""s process includes a first aerobic digestion, followed by chemical treatment of the reaction product with hydrogen peroxide in the presence of ferrous ion in an amount to oxidize the hydrocarbons, and then following the chemical treatment with a second aerobic digestion of the hydrocarbons. The biodigestion steps are achieved with microorganisms, preferably Alcaligenes eutrophus and Pseudomonas sp.
U.S. Pat. No. 5,773,283 to Pierce also discloses the use of microorganisms in bioremediation. Pierce discloses pretreating a chlorinated hydrocarbon contaminant with Fenton""s reagent, followed by introduction of a microorganism for biodegradation.
Specific microorganisms such as Shewanella putrefaciens, a Fe(III)-reducing bacteria, have been linked with direct degradation of a variety of organic contaminants, as reported in U.S. Pat. No. 5,783,088.
Abiotic degradation processes utilizing the powerful oxidative potential of free radicals have been developed for the removal of biorefractory hazardous compounds from industrial effluent and contaminated aqueous environments. These processes generate oxidizing radicals through a variety of mechanisms including UV-potentiated radical propogation (UV/O3, UV/H2O2, UV/TiO2) and transition metal-catalyzed radical formation (Fe(II)/H2O2, Fe(II)/Fe(III)-H2O2, Fe(II)/UV/H2O2). These systems are based on the production of hydroxyl radicals (OH*) which initiate a cascade of reactions that result in the formation of both organic radical intermediates and activated oxygen forms (O2xe2x88x92*, HO2*, OH*). Mineralization of target hazardous compounds is achieved through a series of complex reactions involving radical intermediates until only H2O, CO2, and Clxe2x88x92 remain. The reaction of OH* with organic compounds is unselective and rapid (k greater than 108 Mxe2x88x921sxe2x88x921) and has been used to oxidize a broad range of hazardous organic compounds including pesticides, herbicides, and solvents.
Fenton reaction-generated hydroxyl has been used to treat a wide variety of hazardous organic compounds, including landfill leachates, groundwater contaminated with chlorinated aliphatics and aromatics, drycleaning solvents, nitroaromatic compounds, and azo dyes. Although Fenton reaction-based processes can effectively degrade a wide variety of hazardous organic compounds, continuous addition of Fe(II) and H2O2 is required. In hydroxyl radical technologies operating at a pH of greater than 5, the continuous addition of Fe(II) also results in the generation of large quantities of particulate Fe(III), which contributes to sludge disposal problems.
It is therefore an object of the present invention to design a microbially driven, Fenton reaction-based radical-generating system that operates under neutral pH conditions and requires neither the addition of H2O2 nor the photolysis of Fe(III) to catalyze the oxidative degradation of organic contaminants.
It is a further object of the present invention to combine abiotic and biotic reaction pathways and utilize the H2O2xe2x80x94 and Fe(II)-producing microorganism Shewanella putrefaciens Strain 200P to generate Fenton reagents as metabolic by-products of alternating aerobic and anaerobic respiration.
The present invention is directed to a microbially-driven Fenton reaction for the transformation of organic contaminants. The microbially-driven Fenton reaction combines abiotic and biotic reactions in one system by utilizing the H2O2xe2x80x94 and Fe(II)-producing microorganism Shewanella putrefaciens Strain 200P to generate Fenton reagents as metabolic by-products of alternating aerobic and anaerobic respiration. Microbially-produced Fe(II) and H2O2 react to form OH* via the classic Fenton reaction:
Fe(II)+H2O2, xcfx86OH*+OHxe2x88x92xe2x80x83xe2x80x83(1)
OH* subsequently reacts with the organic contaminant to yield transformation products, which can be further degraded. In the case of pentachlorophenol (PCP), the Fenton reaction product OH* reacts with PCP to yield the hydroxylated transformation products tetrachlorohydroquinone (TCHQ) and tetrachlorocatachol (TCC). Optimal substrate levels are critical for optimizing degradation rates in the microbially-driven system. The process of the present invention allows for the determination of the optimal substrate concentrations (Fe(II), Fe(III), and organic contaminant) and reactor conditions (cell densities, Fe(II) oxidation rates) for maximum rates of contaminant transformation in the microbially-driven Fenton reaction.
Accordingly, the present invention is directed to a process for oxidatively degrading organic contaminants comprising reacting Fe(III) citrate with S. putrefaciens under alternating anaerobic and aerobic conditions to generate Fenton reagents Fe(II) and H2O2, further reacting the Fenton reagents under neutral pH conditions with the organic contaminant to produce degraded transformation products of the contaminant.